dot: name the digraph

* spot/twaalgos/dot.cc: Here.
* NEWS: Mention the change.
* tests/core/alternating.test, tests/core/det.test,
tests/core/dstar.test, tests/core/monitor.test,
tests/core/neverclaimread.test, tests/core/readsave.test,
tests/core/sccdot.test, tests/core/tgbagraph.test,
tests/python/_altscc.ipynb, tests/python/_autparserr.ipynb,
tests/python/alternation.ipynb, tests/python/atva16-fig2a.ipynb,
tests/python/atva16-fig2b.ipynb, tests/python/automata-io.ipynb,
tests/python/automata.ipynb, tests/python/decompose.ipynb,
tests/python/gen.ipynb, tests/python/highlighting.ipynb,
tests/python/ltsmin-dve.ipynb, tests/python/ltsmin-pml.ipynb,
tests/python/parity.ipynb, tests/python/product.ipynb,
tests/python/randaut.ipynb, tests/python/satmin.ipynb,
tests/python/stutter-inv.ipynb, tests/python/testingaut.ipynb,
tests/python/word.ipynb: Adjust test cases.

NEWS

@@ -20,6 +20,12 @@ New in spot 2.5.2.dev (not yet released)
     files produced by "dot -Tsvg" will show those as popups.
     This is also done for state labels of kripke structures.
 
+  - print_dot() will now name the output digraph using the name of the
+    automaton if available, or the empty string otherwise.  (Previous
+    versions used to call all digraphs "G".)  This name appears as a
+    tooltip in SVG figures when the mouse is over the acceptance
+    condition.
+
   - cleanup_parity() and cleanup_parity_here() are smarter and now
     remove from the acceptance condition the parity colors that are
     not used in the automaton.